Starch graft copolymer dispersions are used in pulp and papermaking. They can be used for various purposes, for example for surface sizing compositions or for increasing the dry strength of paper. For example, U.S. Pat. No. 6,426,381 discloses styrene/(meth)acrylate copolymers that can be used for surface sizing. U.S. Pat. No. 6,426,381 further discloses that the obtained dispersions have a particle size of less than 100 nm, even 50 to 90 nm. In practice, however, it has been observed that even if most of the formed polymer particles in the aqueous dispersion are relatively small, there exist also an amount of large polymer particles or polymer agglomerates. This fact is demonstrated by observing a typical particle size distribution for these polymer dispersions. For a dispersion having D(50) value less than 100 nm, D(90) value may already be around 130 nm, and D(99) value around 400 nm. This indicates clearly the presence of large polymer particles or polymer agglomerates.
Uneven particle size distribution in the aqueous dispersion may have negative effects in the end use of the polymer dispersions in papermaking applications. Furthermore, it has been observed that small particle size offers advantages in many applications. Therefore, there exists an interest to prepare polymer dispersions comprising small and even sized particles.